It is a body tissue that has the ability to contract. It shortens and generates force. It relaxes and returns to its original length. Muscles move joints, stabilize the body, move air and food through the organs, act as valves for bladder, bowel and other organs. They control movement of the eyes. They help us express ourselves by changing the shape of our faces (smile, frown, etc).
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2

Are you grinding:
Your teeth? This could be a very simple reason for why your jaw muscles are sore or you could also be clenching a lot. Often associated with stressful life episodes. Don't know about the indigestion, though.
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6

May be sensitive:
If your jaw muscles are irritated from doing a lot of chewing (gum, for example) or from clenching or grinding your teeth, yawning can overstimulate the muscles causing a spasm. This spasm leads to the feeling of tenseness and pain. Try a soft diet for a week or so and see if this helps. You might want to see your dentist as well.
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8

Not a good idea:
In general because over-exercising the jaw muscles (masseters, as in case of brad pitt) could end up overloading the chewing apparatus/jaw joint. I'm afraid brad was just naturally endowed unlike most of us!
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9

Surgery/splint/botox:
If your muscle enlargement is due to clench/grinding - a nightguard may help. Botox can also work in the same situation. Occasionally, the muscles that are enlarged can be sculpted surgically by an oral surgeon. See a board certified oral surgeon for options.
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10

Could be stress:
In today's society, many people are finding themselves under more and more stress. The body often reacts to this by clenching or grinding the teeth. Since the muscles of the jaw are like any other muscle, the more they get 'worked', the larger and more powerful they can become. This will lead to more tension, force, and possible pain and joint problems.
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